After I rig my model with automatic weights, the collar stretches when I turn the shoulders
Almost certainly, you have an issue with the automatic weight calculations, which are never perfect, but often good enough. When they go wrong, they usually cause this behavior, because they work based on vertex proximity to the bones. So some of your vertices which make up the neck, or collar, are contained in the vertex group that is associated with the shoulder. The simplest solution is to go into Weight Paint mode and use brush strokes to remove the collar vertices from the shoulder vertex group. However, this method, while straightforward, is imprecise. Meaning that sometimes, you can miss parts when you go over them with the weight paint brush. A more surefire solution is to select the problem mesh in Object Mode, then go into Edit Mode, click on the Object Data tab in the Properties Panel, and look under the Vertex Weights section for a vertex group with the same name as the upper arm bone (which is effectively the same thing as your shoulder), and make sure it is selected in the list (it will be highlighted if selected). Then, in the 3D View, select all the vertices making up your collar. Go back to the Vertex Weight section in the Properties Panel, and press the Remove button.

Now the collar vertices are no longer part of the shoulder vertex group, and as such, will never move when the shoulder is moved.
Hope this helps.